1. Field of the Invention
The present invention relates to a piezoelectric fan employing a piezoelectric element which exhibits expansion and contraction actions when a voltage is applied thereacross. The present invention also relates to a blow apparatus employing such a piezoelectric fan.
2. Description of the Related Art
Technology that can be applied to a piezoelectric fan is known from, for example, Japanese Utility Model Kokai (Laid-Open) Publication No. SHO 63-140660 entitled "Piezoelectric Actuator". As described in the publication, the piezoelectric actuator is comprised of a metallic intermediate plate electrode having a multiplicity of polygonal apertures extending therethrough, and piezoelectric elements bonded to both surfaces of the intermediate plate electrode.
Reference is made to FIG. 14 hereof which illustrates the principle of a conventional piezoelectric fan employing a piezoelectric element.
Piezoelectric fan 100 is comprised of a square piezoelectric element 101 fixed at one end to a stationary member 102. The piezoelectric element 101 is comprised of a piezoelectric plate 103 and electrodes 104, 104 provided on both surfaces of the piezoelectric plate 103. Extending through the electrodes 104, 104 is a lead 105 connected to an alternating current (AC) source 106. In the piezoelectric fan 100, when an AC voltage is applied across the two electrodes 104, 104, the piezoelectric plate 103 vibrates in directions shown by arrows "a", "a" and generates a wind blowing in a direction of arrow "b". This is the principle of the piezoelectric fan.
With a view to increase wind generating or blow efficiency of piezoelectric fans, the present inventors have investigated various rectangular piezoelectric fans, which are long lengthwise and sidewise.
Reference is now had to FIG. 1A and FIG. 1B showing the manners in which winds are generated by the lengthwise- and sidewise-long piezoelectric fans.
In FIG. 1A, assume that the piezoelectric fan E has a transverse length B and a longitudinal length L, and that the ratio between these is an aspect ratio R. Then, the aspect ratio can be represented by R=B/L.
Stated otherwise, FIG. 1A shows the piezoelectric fan E of small aspect ratio wherein the lateral length B is shorter than the longitudinal length L. As the piezoelectric fan E vibrates in directions of arrows (1), (1), winds blowing in a desired or blow direction of arrow (2) and directions of arrows (3), (3) are generated. Thus, to utilized the piezoelectric fan E of small aspect ratio, it is necessary to install a wall on each side of the fan E since the winds escaping sideways as at (3), (3) are substantial in amount.
FIG. 1B shows the piezoelectric fan F of large aspect ratio wherein the lateral length B is longer than the longitudinal length L. When the piezoelectric fan F vibrates in directions of arrows (4), (4), it generates winds blowing in a desired (blow) direction of arrow (5) and directions of arrows (6), (6). As can be appreciated, in the piezoelectric fan F of large aspect ratio, the wind blowing in the desired or blow direction of arrow (5) is larger in amount than the winds escaping sideways as at (6), (6), which escaping winds are less than those in the piezoelectric fan E of small aspect ratio. In this respect, the piezoelectric fan F of large aspect ratio is more desirable. However, piezoelectric fan F of large aspect ratio encounters problems as described hereinbelow.
Shown in FIG. 2 are lateral and longitudinal mode vibrations of the piezoelectric fan of large aspect ratio. This fan F has a lateral length B and a longitudinal length L. Reference character Vb represents lateral mode vibrations while Vl represents longitudinal mode vibrations. In the piezoelectric fan of large aspect ratio, the lateral mode vibrations Vb are liable to cancel each other, whereby wind generation efficiency is deteriorated.
Further reference is made to FIG. 3 which lists vibration mode analysis data relative to resonant (natural) frequencies f.sub.0 and rates of displacement M generated in a longitudinal mode, a first lateral mode and a second lateral mode at various aspect ratios R. When R=B/L.gtoreq.3.0, resonant frequencies f.sub.0 of longitudinal mode and first lateral mode are within a difference of 25% and rather close to each other. When R=B/L.ltoreq.1.5, resonant frequencies f.sub.0 of the longitudinal mode and first lateral mode are more than 1.7 times away. Accordingly, it is considered that resonance of the first lateral mode become significant when R=B/L.gtoreq.3.0.
Through these studies, the inventors have discovered that it is possible to provide piezoelectric fans with increased wind generation efficiency by employing piezoelectric fans of large aspect ratio (cases 5-7 in FIG. 3) capable of generating more winds blowing in a desired or blow direction while suppressing the lateral mode resonance. This lead to the present invention.